An internal combustion engine is provided with a number of cylinders, each of which is connected to an intake manifold by means of a respective intake valve and to an exhaust manifold valve by means of a respective exhaust valve. The intake manifold receives fresh air (i.e. air from the external environment) via a supply duct regulated by a butterfly valve and is connected to the cylinders by means of respective intake ducts, each of which is regulated by a corresponding intake valve.
It has recently been proposed to introduce a tumble system which is adapted to vary, during operation of the engine, the section of the intake ducts as a function of the engine speed (i.e. the angular speed of rotation of the drive shaft). At low speeds, the passage section of the air through the intake ducts is decreased in order to generate turbulent movements in the suctioned flow of air and in the vicinity of the variation of section, which movements improve the mixing of the air and fuel in the cylinders; as a result of the presence of these turbulent movements which improve mixing, all the fuel injected is combusted with the result that the pollutant emissions generated by combustion are therefore reduced. At high speeds, the passage section of the air through the intake ducts is maximised in order to enable complete filling of the cylinders and thus to enable the generation of the maximum possible power.
In order to vary the passage section of the air through the intake ducts, it has been proposed to dispose a respective butterfly choke valve within each intake duct, which valve rotates about a shaft disposed at the centre of the relative intake duct. This solution is simple and economic to embody, but the presence of the butterfly choke valves in the middle of the intake ducts entails high losses of load in the flow of air when the tumble system is not active because the profile of the butterfly valves is completely immersed in the air flow; obviously, these losses of load reduce the maximum flow of air that can be taken in by the cylinders, thereby reducing the maximum power that can be generated.
To try to remedy the above-mentioned drawback, it has been proposed to provide each cylinder with two intake ducts which are independent from one another and only one of which is provided with a butterfly choke valve. This solution enables better filling of the cylinders at high speeds, but is nevertheless more costly and complex to embody, as two independent intake ducts are provided for each cylinder; moreover, the generation of the turbulent movements at low speeds is not optimal as, for each cylinder, one of the two intake ducts has no variation of section.